linux-kernel - Re: [RFC PATCH 1/3] Unified trace buffer

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Date:	Thu, 25 Sep 2008 14:15:08 -0700
From:	"Martin Bligh" <mbligh@...gle.com>
To:	"Ingo Molnar" <mingo@...e.hu>
Cc:	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	"Steven Rostedt" <rostedt@...dmis.org>,
	"Peter Zijlstra" <peterz@...radead.org>,
	"Martin Bligh" <mbligh@...igh.org>, linux-kernel@...r.kernel.org,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	prasad@...ux.vnet.ibm.com,
	"Mathieu Desnoyers" <compudj@...stal.dyndns.org>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	"David Wilder" <dwilder@...ibm.com>, hch@....de,
	"Tom Zanussi" <zanussi@...cast.net>,
	"Steven Rostedt" <srostedt@...hat.com>
Subject: Re: [RFC PATCH 1/3] Unified trace buffer

>  - save a GTOD+TSC pair at important events, such as to-idle and
>   from-idle, and in the periodic sched_tick(). [ perhaps also save it
>   when we change cpufreq. ]

We did GTOD, but I think it's a bad idea, because NTP moves it.

>  - save the (last_GTOD, _relative_-TSC) pair in the trace entry
>
> with that we have a chance to do good post-processed correlation - at
> the cost of having 12-16 bytes of timestamp, per trace entry.
>
> Or we could upscale the GTOD to 'TSC time', at go-idle and from-idle.
> Which is rather complicated with cpufreq - which frequency do we want to
> upscale to if we have a box with three available frequencies? We could
> ignore cpufreq altogether - but then there goes dependable tracing on
> another range of boxes.

Simple solution: turn off cpufreq whilst tracing is on ;-)

Harder: Keep a timebase and frequency divisor on a per-cpu basis
and calculate your offsets from there. This brings you down to
HPET resolution though
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